The role of antibodies in delayed-type hypersensitivity (DTH) and autoimmune reactions has not been clearly shown. However, vasoactive substances and capillary permeability changes have been shown to play a role. The most important cell involved in the release of vasoactive substances in the vicinity of capillaries is the mast cell. These cells are degranulated by specific antigens reacting with the IgE class of immunoglobulins bound to their Fc epsilon receptors. Degranulation of mast cells is also mediated, although less efficiently, by antibodies of the IgG(1) class. We have shown that Ptx increases the number of lymphocytes containing IgE on their surfaces and that this increase is mainly due to an increase in the number of lymphocytes with Fc epsilon receptors.. We have also shown that hyperimmune antisera to Bordetella pertussis can be depleted of IgG and still retain their ability to protect mice from IC challenge.